In parasites with complex life cycles the transmission of free-living infective stages can be influenced by ambient community diversity, in particular via predation. Here, we experimentally investigated whether parasite density and the presence of alternative prey can alter predation rates on free-living cercarial stages of a marine trematode (Himasthla elongata) by four non-host predators: Semibalanus balanoides (barnacle), Hemigrapsus takanoi (crab), Crassostrea gigas (oyster), and Crangon crangon (shrimp). The four species were tested in four separate experiments, each using the same two-factorial block design, with ecologically relevant cercarial densities (20, 60, 100 or 300 cercariae) and alternative prey (present or absent; fish or algae, depending on species) as main factors and two temporal blocks (day 1 & day 2). Each treatment combination was replicated four times in each block, i.e. 8 replicates for each treatment combination in total. Response variable was the number of cercariae remaining in experimental units. For more detail see publication.